2 of the best places to hunt for alien life just became even more promising

On May 2, an international team of astronomers announced that
they had discovered three Earth-sized exoplanets, all orbiting
the same star (TRAPPIST-1) just 40 light-years from us.

The scientists determined that all three planets are potentially
habitable based on their size and temperature.

Now, the same team has discovered that the two innermost planets
are rocky and have compact atmospheres, making them less like the
hostile planet of Jupiter and more like the rocky planets
of Earth, Venus, and Mars. This makes the prospect of life
lurking in these faraway worlds even stronger. The researchers
published their results today in Nature.

The findings were made just two days after the team announced
that it had found the planetary system.

A promising place to detect alien life

Systems like this are promising places to detect alien life,
Michaël Gillon, lead author of the paper presenting the
discovery, said in an ESO press
release .

The host star is an ultracool dwarf star — a type of cool, red
star. Most of the time these stars are too small and faint to be
detected by optical telescopes, and this star is no exception.

"Why are we trying to detect Earth-like planets around the
smallest and coolest stars in the solar neighborhood? The reason
is simple: systems around these tiny stars are the only places
where we can detect life on an Earth-sized exoplanet with our
current technology,” Gillon said.

Because of their closeness to the star, the two innermost planets
are likely tidally locked, with one side always facing the star
and the other always facing away.

Although the sides facing the star would be too hot to host any
lifeforms and the sides facing away would be too cold and dark,
the planets might contain "sweet spots." If the planets have
atmospheres or even possibly oceans, heat from the star might be
more evenly distributed, creating regions that just might be
suitable for life.

A double transit

ESO/M. Kornmesser/N. Risinger
(skysurvey.org)

To further investigate the promising planets, the researchers
pointed NASA’s Hubble Space Telescope at TRAPPIST-1 just in time
to catch a double transit, which is when two planets pass in
front of the same star at almost the same time.

"We thought, maybe we could see if people at Hubble would give us
time to do this observation, so we wrote the proposal in less
than 24 hours, sent it out, and it was reviewed immediately,"
Julien de Wit, a postdoc in MIT's Department of Earth,
Atmospheric and Planetary Sciences, said in an MIT press release.
"Now for the first time we have spectroscopic observations of a
double transit, which allows us to get insight on the atmosphere of both planets
at the same time."

The dips in starlight that occurred when the planets crossed in
front of the star indicated that the planets have compact
atmospheres, which are more suitable to life.

"We can say that these planets are rocky. Now the question is,
what kind of atmosphere do they have?" de Wit said. "The
plausible scenarios include something like Venus, where the
atmosphere is dominated by carbon dioxide, or an Earth-like
atmosphere with heavy clouds, or even something like Mars with a
depleted atmosphere. The next step is to try to disentangle all
these possible scenarios that exist for these terrestrial
planets."